1. Field of the Invention
The present invention relates to an impedance compensation circuit in a speaker driving system and, more particularly, to an impedance compensation circuit which can prevent a change in drive state caused by a variation in internal impedance inherent in a speaker, a variation in impedance of a connecting cable or the like for connecting the speaker and a driver, and changes in such impedances due to a change in temperature.
2. Description of the Prior Art
In general, an electromagnetic converter (dynamic electro-acoustic converter) such as a speaker obtains a driving force by flowing a current i through a coil (e.g., a copper wire coil) in a magnetic gap of a magnetic circuit. If a conductor length of the copper wire coil is represented by l and an intensity of a magnetic field of the magnetic gap is represented by B, a driving force F appearing at the copper wire coil is given by: EQU F=B.multidot.l.multidot.i
In constant-current driving, since an electromagnetic damping effect cannot satisfactorily function, a constant-voltage driving system is normally employed for driving a speaker system. In the constant-voltage driving system, the current i flowing through a voice coil changes depending on an internal impedance inherent in a speaker and an impedance of a connecting cable with a driver side. Therefore, the driving force F appearing at the copper wire coil varies or changes depending on a variation of the speaker or connecting cable or changes in impedances caused by a change in temperature.
The above-mentioned electromagnetic conversion system generally has a motional impedance. A resistance of the voice coil or the connecting cable also serves as a damping resistance of this motional impedance. For this reason, when the internal impedance of the speaker or the impedance of the connecting cable varies, the damping force to the voice coil also varies. When these impedances vary upon a change in temperature, this damping force is also changed.
A negative impedance driving system which can realize a larger driving force and damping force than the constant-current driving system has been proposed. In this system, a negative output impedance is equivalently generated in a driver, and a speaker as a load is negative-impedance driven. In order to equivalently generate the negative output impedance, a current flowing through the voice coil of the speaker as the load must be detected. For this purpose, a detection element is connected in series with the load. In the system performing the negative-impedance driving, an internal impedance of the load is apparently eliminated or canceled by the equivalently generated negative output impedance, thus achieving both the large driving force and damping force at the same time.
This system will be briefly described below with reference to FIGS. 2(a) and 2(b). In FIG. 2(a), Z.sub.M corresponds to a motional impedance of an electromagnetic converter (speaker), and R.sub.VO corresponds to an internal resistance R.sub.V of a voice coil as a load. As shown in FIG. 2(b), the internal resistance R.sub.V is eliminated by a negative resistance -R.sub.A equivalently formed at a driver side, and an apparent driving impedance Z.sub.A is given by: EQU Z.sub.A =R.sub.V -R.sub.A
In this case, when Z.sub.A becomes negative, the operation of the circuit becomes unstable. Therefore, in general, R.sub.V .gtoreq.R.sub.A.
However, in the negative-impedance driving system described above, it is difficult to keep constant the driving impedance for the motional impedance with respect to variations in internal impedance of the speaker or impedance of the connecting cable or a change in internal impedance caused by a change in temperature. More Specifically, in the circuit shown in FIGS. 2(a) and 2(b), if the equivalent negative resistance -R.sub.A is kept constant, a ratio of an influence caused by a variation in internal impedance of the speaker or impedance of the connecting cable or a change caused by a change in temperature becomes larger than that in the above-mentioned constant-voltage driving system.
There is no conventional means for positively preventing an adverse influence caused by a variation in load impedance or a change in temperature which is particularly conspicuous in the negative-impedance driving system.